<img src="" alt="" class="plbg">
<script>
    var bgchoice = "<%= site.customConfig.bgchoice %>";
    if(bgchoice=='default')
        var plbgurl = "<%= site.customConfig.mobileplbgimage %>"
    else  
        var plbgurl = "<%= site.customConfig.mobileplbgimagelink %>"
    $(".plbg").attr("src",plbgurl);
    $(".plbg").fadeIn(200);
</script>